1. Field of the Invention
This invention relates to a catalyst for the purification of diesel engine exhaust gas.
2. Description of the Prior Art
In recent years, those fine particulates which are entrained particularly by diesel engine exhaust gas (which mainly comprise solid carbon particles, particles of sulfur type compounds such as sulfates, and liquid or solid macromolecular hydrocarbon particles and which will be hereinafter referred to collectively as "fine particulates") have been posing a serious problem before environment and hygiene. The reason for the problem is that these fine particulates very rarely have diameters not more than 1 .mu.m and, therefore, are liable to be drifted in the air and inhaled into human bodies. Thus, measures are being pushed toward increasingly exacting regulations which concern the release of such fine particulates from diesel engines into the ambient air.
The trend of diesel engines toward use of increasingly high pressure for the injection of fuel and control of increasingly accurate timing of the injection of fuel has already decreased to a certain extent the amount of minute particulates discharged from diesel engines. This decrease, however, falls short of being fully satisfactory. The component of the fine particulates which mainly comprises liquid macromolecular hydrocarbons soluble in organic tractions (SOF) cannot be removed in spite of such improvements heretofore attained in diesel engines as mentioned above. These improvements have rather brought about an increase in the proportion of the SOF in the minute particulates. Since the SOF contains such harmful substances as cancerogenic substances, the removal of the SOF has become as important a task as the removal of fine particulates.
As respects means to remove minute particulates, studies are under way to establish the feasibility of a catalytic method which comprises seizing minute particulates from diesel engine exhaust gas by the use of a catalyst having a catalytic substance capable of effecting combustion of carbonaceous particles deposited on a three-dimensional refractory structure such as ceramic foam, wire mesh, metal foam, closed type ceramic honeycomb, open flow type ceramic honeycomb, or metal honeycomb and removing the seized carbon type particles by either utilizing such exhaust gas (with respect particularly to composition and temperature) as is freshly discharged from a diesel engine operated under the condition of normal travel or using such heating means as an electric heater.
Generally, the catalyst to be used for the purification of diesel engine exhaust gas is desired (a) to provide highly efficient removal by combustion of such harmful substances as unburnt hydrocarbons and carbon monoxide as well as carbonaceous type particles in a wide temperature range from a low temperature zone upward, (b) to possess only a meager ability to oxidize sulfur dioxide (SO.sub.2) generated from the sulfur component contained richly in the gas oil used as the fuel into sulfur trioxide (SO.sub.3) and repress the formation of sulfates (i.e. sulfur trioxide and sulfuric acid mist which result from the oxidation of sulfur dioxide), and (c) to abound in the so-called high-temperature durability, namely the ability to tolerate continuous operation under high load.
Various proposals have been heretofore made which are aimed at effecting the removal by combustion of carbonaceous particles with exalted efficiency. For example, as platinum group element-containing catalysts, JP-A-55-24,597 discloses a rhodium (7.5%)-platinum alloy, a platinum/palladium (50/50) mixture, a composite having palladium deposited on tantalum oxide or cerium oxide, and a mixture of palladium with not more than 75% by weight of platinum. They are claimed to be effective in catalyzing also the removal of SOF.
JP-A-61-129,030, JP-A-61-149,222, and JP-A-61-146,314 disclose such catalyst compositions which use palladium and rhodium as main active components and additionally incorporate therein alkali metals, alkaline earth metals, copper, lanthanum, zinc, and manganese. JP-A-59-82,944 discloses such a catalyst composition as combines at least one member selected from the group consisting of copper, alkali metals, molybdenum, and vanadium with at least one member selected from the group consisting of platinum, rhodium, and palladium.
Besides, an open type honeycomb-shaped noble metal oxide catalyst having through holes formed therein parallelly to the flow of gas has been reported (SAE Paper, 810263) as a catalyst for the removal of SOF from the diesel engine exhaust gas.
Indeed, the conventional catalysts mentioned above are invariably effective to a certain extent in removing by combustion the carbonaceous particles and in removing SOF. They, however, are highly effective in oxidizing sulfur dioxide and therefore are at a disadvantage in increasing the amount of sulfates to be formed, rather degrading the efficiency of the removal of minute particulates as a whole, and consequently suffering the sulfates to cause a new problem of environmental pollution.
A catalyst which possesses the aforementioned properties of (a) to (c) required of a catalyst used for the purification of diesel engine exhaust gas and manifests an ability to effect ample removal of the SOF remains yet to be developed.
A primary object of this invention, therefore, is to provide a catalyst for the purification of diesel engine exhaust gas which is capable of efficiently removing minute particulates entrained by the diesel engine exhaust gas.
Another object of this invention is to provide a catalyst for the purification of diesel engine exhaust gas which possesses an ability to remove by combustion such harmful substances as unburnt hydrocarbons and carbon monoxide as well as carbon type particles in the diesel engine exhaust gas in a wide temperature range from a low temperature zone upward, exhibits only a low capacity for oxidizing sulfur dioxide, and represses the formation of sulfates.
Yet another object of this invention is to provide a catalyst for the purification of diesel engine exhaust gas which is capable of removing the SOF in the diesel engine exhaust gas with high efficiency.
Still another object of this invention is to provide a catalyst for the purification of diesel engine exhaust gas which is excellent in high-temperature durability and, therefore, can be effectively incorporated in a diesel car without entailing any problem from the practical point of view.